This invention relates generally to pressure transducers of the strain gage type and more particularly to pressure transducers having zero temperature coefficient (zero TC) compensation. A strain-gage type pressure transducer converts a physical displacement into an electrical signal. Strain-gage type pressure transducers have been manufactured using integrated circuit technology for numerous applications. A typical miniature pressure transducer includes a thin silicon diaphragm into which resistors are diffused or implanted and then connected to form a Wheatstone Bridge circuit. While these pressure transducers offer many advantages, such as their use as disposable blood pressure transducers, they do not give consistent results in environments with varying temperatures. Inconsistent results due to temperature variation is caused by the different thermal expansion coefficients of the two or more layers of the composite forming the diaphragm, slight variations in the temperature coefficients of resistance (TCR) of individual bridge elements, thermal gradients between elements, or any combination of these causes.
Prior art schemes for providing zero TC compensation include on-chip and off-chip resistive compensation. Since the silicon diaphragm transducer is miniaturized, many manufacturers use circuitry on additional chip(s) which is electrically connected to the diaphragm chip to provide zero TC compensation. While this scheme can provide acceptable zero TC compensation in many cases, the use of off-chip circuitry is not always convenient for applications requiring greater miniaturization and may not adequately compensate a unit in which a temperature differential exists between the diaphragm and compensation chips.
A prior art method of providing on-chip zero TC compensation is shown in FIG. 2. In this pressure transducer, thin-line aluminum resistors are deposited on the edge of the diaphragm chip, outside the diaphragm and electrically connected in the bridge circuit. Aluminum has a temperature coefficient of resistance (TCR) of about 3900 ppm/degree C., while the diffused bridge resistors would typically have a design value between 1000 and 2000 ppm/degree C. The effective TCR of the Wheatstone Bridge arms that include the aluminum resistors would then be greater and as a result a zero TC would be induced to compensate the transducers. While the structure of FIG. 2 is useful for providing zero TC, it is expensive to manufacture and requires tight processing control. For example, the amount of zero TC compensation provided by the aluminum resistors of FIG. 2 depends on the following parameters: aluminum resistor TCR and ohmic value (i.e. line width, thickness, and resistivity); TCR of the diffused bridge resistor; and bridge impedance. Of these factors, variations in line width or thickness provide the major shifts in compensation variables. In addition, since the compensation resistors are in series with the bridge resistors, bridge balance is coupled with zero TC compensation and thus will also shift with variations in the value of the aluminum resistors.
Therefore, it is an object of the present invention to provide a pressure transducer having zero TC compensation which is independent of bridge balance.
It is another object of the present invention to provide a pressure transducer having zero TC compensation which is simple to manufacture and requires a minimum of processing control.
It is yet another object of the present invention to provide a pressure transducer with improved on-chip zero TC compensation.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.